Graduate Thesis Or Dissertation

Transport in silicon metal oxide semiconductor quantum dots

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  • Herein, a program of research is detailed related to transport through the Si metal oxide semiconductor (MOS) quantum dots fabricated in a process flow compatible with modern ULSI (ultra large scale integrated circuit). Silicon quantum dots were fabricated by placing split gates within a MOSFET structure. Quantum dots of several sizes and geometries were fabricated by this process for the purpose of investigating the effects of size and shape on quantized transport through the dots. The transport properties of the different quantum dot sizes and shapes were investigated at low temperatures, and compared to normal MOSFETs fabricated by the same technology. Equilibrium measurements with the device biased in the regime from the onset of weak inversion to just past the onset of strong inversion revealed strongly oscillatory behavior in the tunneling conductance. The conductance peaks appear to map an energy level spectrum in the dot as the inversion and depletion gates are separately swept. Symmetric devices, biased both symmetrically and asymmetrically, show two groups of "branches" which evolve with different slopes in the V[subscript Inv]-V[subscript Depl] plane. An asymmetric device studied shows three groups of branches. In addition, a fine structure is observed in the conductance peak behavior of two devices. This apparent energy level structure is compared to the body of literature on the so-called artificial atoms, as well as self-consistent three dimensional quantum mechanical solutions for the energy levels in the same dot structure, which qualitatively agree with the overall slope of the observed data. However, the calculations reveal only the multiple sets of slopes when asymmetrically biased. These multiple slopes are postulated to arise due to the splitting of the degenerate states of the symmetric structure as the bias makes the structure increasingly asymmetric. Finally, a simplified model is presented which shows how slight asymmetry in the dot confining potential can give rise to both a fine structure and multiple slopes in the branches, and several alternative mechanisms are presented to explain the origin of the fine structure observed.
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